Optical tables are used to support sensitive instrumentation, and thus require precise leveling and freedom from vibration. In a typical optical table, the optical table top is supported by a suspension system or table mount. Three types of suspension systems are known in the art, namely rigid mounts, pneumatic self-leveling mounts, and elastomeric or pneumatic mounts augmented by very expensive electromagnetic suspension systems.
The first mentioned suspension systems, rigid mounts, although the least expensive, are only suitable for applications where leveling is required, but not vibration isolation. The third mentioned suspension systems, elastomeric or pneumatic suspension systems augmented by expensive electromagnetic suspension systems, are very expensive, and are tailored to the specific needs of a small segment of the active vibration cancellation market. Therefore, the most commonly used suspension systems are the pneumatic self-leveling suspension system.
The pneumatic self-leveling suspension systems typically comprise sensors, pneumatic controls, and structural supports. Each of the vibration isolators in the system act as air springs in that they utilize the compressibility of air contained in a chamber, a sealing element, and a piston to produce the characteristics of a low frequency spring. Thus the pneumatic self-leveling system can accommodate varying loads without major deflections by varying the air pressure in the air chamber.
However, one problem with the pneumatic self-leveling suspension systems is that the systems are incapable of isolating the table from disturbances either generated directly on the table, or otherwise directly impacting the table. For example, rapid load shifts on the table may cause large direct disturbances, such that the table is no longer level. As another example, rotating or moving components on the optical table top will cause the table to respond more than a comparable table placed on rigid mounts. More specifically, the small disturbances on the table may cause the table to vibrate in unison with the disturbances, while the table appears to remain level. Moreover, if not properly designed, the system itself may generate a significant amount of vibration as a result of the contacting forces on the table by mechanical lever arms, or allow the passage of vibrations by the hysteresis of the thin rubber skins used as rolling diaphragms in the system.
Attempts have been made to improve the pneumatic suspension systems. For example, efforts have been made to reduce the effect of changes in the static load on the systems, thus creating zero deflection pneumatic suspensions. The zero deflection pneumatic suspensions make small adjustments to the pneumatic flexibility of the suspensions. The changes in flexibility are made by adjusting the pressure in the load bearing. The pressure adjustments are continued until the suspended entity to the position and/or orientation it occupied prior to the change in the static suspension load. The problem however, with such known types of the zero deflection devices is caused by their slow response. The slowness of the response to changes in static load renders the systems ineffective for many applications.
Efforts have also been made to reduce the effect of the dynamic forces on the suspended entities. Systems have thus been developed using transducers to ascertain the characteristics of the disturbance forces, wide bandwidth analog or high-speed digital techniques to determine cancellation forces proportional to the additive inverses of the dynamic forces, and transducers again to impart the cancellation forces on the suspended entities. However, several problems existed with the transducers used in these known systems. For one, the transducers impart the cancellation forces on the suspended entities at points different than the points of suspension, reducing the effectiveness of the system. Secondly, the cancellation forces created by the system are too small in comparison to the forces needed to minimize the suspended entity's response to sudden dynamic forces created by movement of components connected to the suspended entity.
Therefore, a need exists for an optical table vibration cancellation and active leveling system that provides a quick response to changes in static and dynamic load. Also, a need exists for an optical table vibration cancellation and active leveling system that provides sufficient forces at the points of suspension to minimize the effect of sudden forces exerted on the optical table.